I know exactly what you mean. Even know I haven't applied a friend of mine did apply last year to Birmingham Conservatoire. What you need is at Grade 8 in your principal study e.g. Piano, Violin, Trumpet etc. It's now extra helpful now because you can use them for UCAS points if your A Levels/GCSE's aren't enough.
For example if you applied to the university of your choice with a C in Music (minimum) with your other A level subjects at a average pass, and it's almost enough, your Grade 8 in your principal instrument at a Pass with Distinction and any other Grade exams above Grade 5 in other instruments, it will be very significant to show both the qualifications for the tariff points and also your performing standards which will be what they will be looking for.
In most cases they will set their requirements for A level Music at an A/B (recommended) and another subject that will be required academically and your Grades for your instrument of study. The second instrument of study is what you can do at Uni as an mandatory option which means as well as playing the flute, you have to chose another instrument to study, but you can choose which one you want rangeing from Harpsichord, Organ, etc.
It depends which University/Institution you apply to, the more reputation and depth in the course e.g. Cambridge, Oxford, London etc. they will be asking for the best qualified because that's is what those places will be looking for: they are looking for the best in the country. You will face a lot of very bright people with extraordinary talent. Cambridge or someplace else it really doesn't matter because you will face alot of competition coming from alot of young people.
Sorry if I was scaring you there, I didn't meen to.
If you have anything you wish to ask, don't hesitate to say so.
Hope that's helped you.